I. Field of the Invention
The present invention relates to soldering by condensation heat transfer using the latent heat of fluid perfluorinated organic compounds to melt solder alloys for attaching an electronic component to a printed circuit board and the like. More particularly, the present invention relates to such soldering techniques using certain high boiling chemically inert fluid perfluorinated polyethers.
II. Prior Art
Vapor phase condensation reflow soldering is a well known procedure for mass soldering of wire terminal pins to printed circuit boards and the like. For example, U.S. Pat. No. 3,866,307 discloses a condensation reflow soldering technique. In simple terms, reflow soldering is done by first applying a predetermined amount of solder paste or cream to a surface to be soldered and heating the assembly to melt the solder and to thus form the joint. Vapor phase soldering utilizes the latent heat of condensation of vaporized fluid to melt the solder and requires the fluid, among other things, to be exceedingly chemically inert, to have a necessary boiling point range, and to be safe and facile to handle. Certain perfluorinated organic compounds, that is to say, organic compounds which are fully fluorinated, such as alkanes, ethers and tertiary amines, are well known versatile fluids that possess the just-mentioned properties and thus have found use in vapor phase soldering of electronic components. These known compounds, however, possess certain limitations when used as fluids in vapor phase soldering.
From a chemical standpoint those perfluorinated organic compounds which have been used commercially as vapor phase soldering fluidscan be generally classified as (1) perfluoro-tertiary amines (PTA), such as perfluorotriamylamine, (2) perfluoro-perhydrophenanthrene (PPH) and (3) mixed perfluoropolyethers (MPE). MPE's are random copolymers having the formula ##STR1## wherein m/n is about 40/1.
It is known that PTA's can be produced by electrochemical fluorination of the corresponding tertiary amine and suffer the drawback of possessing less thermal stability and chemical stability than desired. During use as a vapor phase soldering fluid very corrosive hydrofluoric acid forms due to degradation of some less stable PTA's. PTA's also tend to break down after long thermal exposure into detectable quantities of the toxic gas perfluoroisobutylene.
Although there is less concern of toxic and corrosive decomposition of PPH as compared to PTA, soldering fluxes dissolve in PPH to an undesirable high level. The build up of flux in the PPH fluid results in the deposition and subsequent decomposition of the flux on the heater elements used to vaporize the PPH fluid. Furthermore, PPH fluids are consumed during vapor phase soldering in an excessive amount. PPH fluids suffer excessive losses from vapor phase soldering equipment by evaporation and diffusion. These losses of expensive PPH fluid represent an expense that should be minimized.
MPE's are made by a polymerization process using perfluoroalkylene epoxides. Prior art MPE's are terminated with various terminal groups. Because of their manner of manufacture, MPE fluids are composed of mixtures of oligomers and possess a relatively wide boiling point range rather than a more desirable narrow range which PPH and PTA fluids desirably exhibit.